Process of reducing the acid neutralization number of crude petroleum



United States This invention relates to new and useful improvements in processes for the refining of crude petroleum to reduce the amount of free acid present therein.

Many crude oils contain suflicient quantities of naphthenic acids to make it necessary to modify refining procedures in order that products such as lubricating stocks and lubricating oils be essentially free of acids. In the refining of crude petroleum, which contains substantial amounts of naphthenic acids, there is a problem of excessive corrosion in the distillation equipment due to the production of hydrogen chloride by reaction of naphth enic acids in the petroleum with trace amounts of inorganic chlorides in solution. In the past, attempts have been made to remove naphthenic acids from crude petroleum or from petroleum fractions at various points in the refining processes or to convert the naphthenic acids into neutral salts or esters. Prior methods dealing with naphthenic acids have included neutralization by alkali, catalytic hydrogenation of the carboxylic groups, extraction with various phenols, alcohols, and other solvents, with or without neutralization or saponification, and claytreating. Numerous counter-current solvent-extraction processes have been proposed for use in refining crude petroleum oil. One process which has been used utilizes an extracting solution consisting of an alkylolamine dissolved in an aliphatic alcohol to react with and remove the naphthenic acids. This method, however, produces sludge which serves no useful purpose and presents a disposal problem in many instances. Another method which has been used for reducing the acidity of crude petroleum has utilized treatment with aqueous alkali to neutralize naphthenic acids, followed by separation of the aqueous solution of alkali metal naphethenate salts. This process, however, has been ineffective with many types of crude petroleum which contain high-molecularweight naphthenic acids because the alkali-metal naphthenate salts are oil-soluble and cannot be removed by washing with water.

It is therefore one object of this invention to provide an improved process for reducing the acid neutralization number of crude petroleum containing substantial amounts of naphethenic acids which form alkali-metal salts which are oil-soluble.

Another object of this invention is to provide an improved process for refining crude petroleum containing substantial amounts of naphthenic acids in which the naphthenic acids are converted first into oil-soluble alkalimetal salts and subsequently into naphthenate esters.

A feature of this invention is the provision of an improved process for reducing the acid neutralization number of crude petroleum containing substantial amounts of naphthenic acids by treatment with aqueous alkali followed by reaction with a lower alkyl sulfate or halide atent ice y K; g

to convert the naphthenic acids into oil-soluble esters and permit the removal of oil-insoluble by-product salts.

Another feature of this invention is the provision of an improved process for reducing the acid neutralization number of crude petroleum containing substantial amounts of naphethenic acids, the alkali-metal salts of which are oil-soluble, in which the petroleum is treated with aqueous alkali and then reacted with a lower alkyl sulfate or halide prior to treatment of the petroleum in the desalter, which removes by-product salts and naturallyoccurring salts from the treated petroleum.

Other objects and features of this invention will become apparent from time to time throughout the specification and claims as hereinafter related.

This invention is based upon our discovery that the free naphthenic acid content of crude petroleum, as measured by the acid neutralization number, can be reduced drastically by contacting the petroleum with a solution of an alkaii-metal hydroxide, then with an alkyl sulfate or halide, and finally desalting the petroleum to remove the by-product and naturally-occurring salts. By this means, the acid neutralization number of crude petroleum can be substantially reduced with the result that products resulting therefrom also show an appreciable reduction in acid number, and corrosion of equipment during distillation is minimized. This treatment of the crude petroleum with alkali-metal hydroxide solution, followed by treatment with alkyl sulfate or halide, does not result in the removal of the naphthenic acids, but converts the naphthenic acids into neutral esters which have no acid neutralization number and are harmless for practically all purposes for which the petroleum oils are used. This treatment avoids any increase in ash content of the petroleum' resulting from the neutralization of naphthenic acids with alkali-metal hydroxides which form oil-soluble naphthenates not removed by the aqueous'solution. By utilizing this two-step treatment, it is possible to convert the naphethenic acids into oil-soluble neutral esters, and to remove the by-product salts, without producing oilsoluble alkali naphthenate salts. which tend to increase the ash content of the oil.

For the purposes of this invention, the crude petroleum is treated with a quantity of alkali-metal hydroxide in solution, and with an alkyl sulfate or alkyl halide, both of which are at most only slightly greater than the stoichiometric quantities required to convert the naphthenic acids to esters according to the following equations (using sodium hydroxide and dimethyl sulfate or methyl chloride for purposes of illustration):

01' (2b) RCOONa+CH Cl=RCOOCH +NaCl.

The acid neutralization number of untreated crude pe-' troleum is a measureof the free acid content in terms of the milligrams of potassium hydroxide required to neutralize 1 gram of oil. From-this value, it is a simple matter to calculate the stoichiometric amounts of sodium hydroxide (or other alkali-metal hydroxide) and dimethyl sulfate or methyl chloride required for treating the oil, since 1 mol of each is required per mol of acid and the amounts of sodium hydroxide and dimethyl sulfate or methyl chloride show a simple molecularweight relationship to the saponification number. In carrying out this process, the petroleum is treated in each step with the indicated reagent at a temperature of about 150l80 F., although lower temperatures can be used, as well as somewhat higher temperatures. If lower temperatures are used, the reaction time is prolonged to some extent and the efficiency of the process is lowered. If higher temperatures are used, it may be necessary to use pressure equipment to avoid vaporization of the esterifying agent. The crude petroleum may be diluted, if desired, with a solvent in order to improve the contacting efiiciency.

In carrying out this process, the first step has as its goal the maximum conversion of naphthenic acids into alkali-metal naphthenates. This maximum conversion is effected by carrying out the reaction with the alkalimetal hydroxide (which may he sodium hydroxide, potassium hydroxide, lithium hydroxide, etc.) in aqueous solution. Preferred concentration is in the range of about 5-20% wt. of the metal hydroxide in water. The time of contact with the aqueous alkali may vary from a few minutes to as much as one hour, depending upon the concentration of naphthenic acids and the degree of agitation or intimacy of contact. Good conversion of the naphthenic acids in petroleum into metal naphthenates is difficult to attain with solid metal hydroxides, but is attained quite easily with aqueous solutions. After the reaction with aqueous alkali is complete, the alkalimetal naphthenates are reacted in situ with an alkyl sulfate or alkyl halide (preferably a lower alkyl compound, such as methyl or ethyl sulfate, chloride, bromide, iodide, etc.). The alkyl sulfate or halide is mixed with the petroleum containing alkali-metal naphthenates in solution, and allowed to react for a period of 1 to minutes, orlonger. Reaction of the alkalimetal naphthenates with the alkyl sulfates of alkyl halide is far more rapid than the reaction of alkalimetal hydroxide with an alkyl sulfate or alkyl halide. In carrying out this process, both the metal hydroxide solution and alkyl sulfate or halide may be added either batchwise or continuously, the preferred method of addition being by means of a proportioning pump. If desired, the alkyl sulfate or halide may be added immediately after the metal hydroxide solution, but a somewhat better conversion into esters is obtained when the alkyl sulfate or halide follows the metal hydroxide solution after an interval sufficient for optimum conversion of the naphthenic acids into metal naphthenates.

The following non-limiting examples are illustrative of the scope of this invention.

EXAMPLE I In a series of experiments, an acidic lubricating oil extract (having an acid neutralization number of 6.5), obtained by phenol extraction of lubricating oil stock, was treated in accordance with the instant process. While these experiments demonstrated the applicability of the process to finished oils, it is more feasible to utilize the process in reducing the acid number of crude oils prior to treatment of crude oil in the desalter.

In each of these experiments, 200 g. of the acidic oil extract was stirred mechanically with a 5% wt. sodium hydroxide solution for the time indicated and at 170 F. Then dimethyl sulfate was added and the mixture was stirred for an additional period, as indicated in the tabulation of results. The lower aqueous layer was withdrawn and the acid neutralization number of the treated oil was determined. In order to avoid any possible difliculty with sodium naphthenate which might still be present in the oil, the treated oil was acidified with dilute sulfuric acid and the sulfuric acid layer was drawn off while the material was kept hot. The oil then was washed repeatedly with hot water until the washings were entirely free of mineral acid, as

determined by methyl orange indicator. The acid neutralization number of oil was determined in accordance with A.S.T.M. Method D-974. The results of these experiments are set forth in the following table:

Table l ACID NUMBER REDUCTION OF EXTRACT OIL Mols of Mols of Rednc- N aOH Contact Dirnethyl Contact Acid tion In Added Time of Sulfate Time Number Acid Run Per Mol NaOH Added With Number No of N apn- With Per M01 of Dirncthyl Treated Due to theme The Oil N aphthenic Sulfate Oil Treat- Acid (Min) Acid (Mln.) ment (percent) Ammonium hydroxide was used instead of sodium hydroxide in this run.

2 Added simultaneously.

From these experiments, it is seen that a substantial reduction in acid neutralization number can be ac complished following our method of treatment.

EXAMPLE II The results of our experiments are applied to the treatment of acidic crude oils by carrying out neutralization and esterification steps prior to treatment of the crude in a desalter to remove water and dissolved salts. A 1000-g. sample of a Texas crude oil having an acid neutralization number of 1.5 is treated in accordance with our invention. To this sample of crude, there is added 7.1 ml. of 15% aqueous sodium hydroxide with agitation. The mixture is agitated at a temperature of 180% F. for a period of 1 hour. At the end of this time, 3.4 g. of dimethyl sulfate is added, representing 1 mol of dimethyl sulfate per mol of naphthenic acid, and the mixture is agitated for 10 minutes. After receiving this treatment, the crude oil is passed into an electric desalter, where the water-in-oil emulsion is broken, and water settles out as a separate layer containing dissolved, naturally-occurring salts and by-product sodium methyl sulfate. In carrying out this phase of the process, any one of the desalters, which are conventionally used in breaking petroleum emulsions and removing salts, is satisfactory. Examples of these de salters are well known in the art and can be found in Nelson, Petroleum Refinery Engineering, 4th edition, McGraw-Hill Book Company, pages 265-268. Other types of desalter equipment, e.g., settling tanks, filtration beds, etc., may be substituted for the electric desalter in our process for the reduction of acid numbers in crude oils. After removal of by-product salts and naturally-occurring salts from the crude oil, the acid number is 0.1.

EXAMPLE III A 2000-g. portion of Texas crude oil having an acid neutralization number of 1.5 is treated with 20.0 ml. of a 15% aqueous solution of potassium hydroxide. The mixture is agitated at a temperature of F. for a period of one hour to allow complete neutralization of naphthenic acids. To this mixture, there is added 3.5 g. of ethyl chloride (1 mol per mol of naphthenic acid), and agitation is continued at a temperature of 160 F. for about 10 minutes. At the end of this time, the crude oil is passed through a sand filter and then through an electric desalter, and is allowed to settle and separate into aqueous and oil phases. The aqueous phase is withdrawn and contains by-product salts and naturally-occurring salts from the crude oil. The crude oil has an acid neutralization number of 0.2 at the end of this period of treatment.

While we have described this invention fully and completely with special emphasis upon several preferred embodiments, we wish it to be understood that this process may he used in the treatment of any crude oil having a substantial content of naphthenic acids, and that the process may use any alkali-metal hydroxide for neutralization of the acids and any lower alkyl sulfate or halide for conversion of the metal naphthenates to naphthenate esters. In order to carry out this process economically, it is necessary to treat the crude oil prior to its treatment in a desalter, so that the lay-product salts are removed with the naturally-occurring salts from the crude oil after treatment.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of reducing the acid neutralization number of crude petroleum containing substantial amounts of naphthenic acids which comprises treating the petroleum with aqueous alkali to convert the naphthenic acids to naphthenate salts, reacting the naphthenate salts in situ with an esterifying agent selected from the group consisting of lower alkyl sulfates and lower alkyl halides to produce a lower alkyl naphthenate and by-product salts selected from the group consisting of alkali halides and alkali lower alkyl sulfates, and treating the petroleum in a desalter to remove hy-product salts and naturally occurring salts.

2. A method according to claim 1 in which the aqueous alkali used is an aqueous solution of an alkali-metal hydroxide in an amount just sufiicient to neutralize substantially all of the naphthenic acids, and the esterifying agent is dimethyl sulfate.

3. A method according to claim 1 in which the aqueous alkali used is an aqueous solution of an alkali-metal hydroxide in an amount just suflicient to neutralize substantially all of the naphthenio acids, and the esterifying agent is diethyl sulfate.

4. A method according to claim 1 in which the aqueous alkali used is an aqueous solution of an alkali-metal hydroxide in an amount just sufilcient to neutralize substantially all of the naphthenic acids, and the esterifying agent is methyl chloride.

5. A method according to claim 1 in which the aqueous alkali used is an aqueous solution of an alkali-metal hydroxide in an amount just sufficient to neutralize substantially all of the naphthenic acids, and the esterifying agent is ethyl chloride.

6. A method according to claim 1 in which the desalter used is an electrical emulsion-breaker.

7. A method according to claim 1 in which the alkali treatment and esterification are each carried out at about 150-180 F.

8. In the refining of crude petroleum containing salts in the form of an emulsion and containing a substantial amount of naphthenic acids which form oil-soluble salts, in which the petroleum is treated in a desalter prior to distillation, the improvement which comprises treating the petroleum with aqueous alkali, prior to treatment in the desalter, to neutralize the acids and produce alkali naphthenate salts, reacting the naphthenate salts in situ with an esterifying agent selected from the group consisting of lower alkyl sulfates and lower alkyl halides, to pro duce lower alkyl naphthenate esters and lay-product salts, and then treating the petroleum in the desalter to remove the salts therefrom.

9. In a method according to claim 8 in which the petroleum is treated with a 5-20% aqueous solution of an alkali-metal hydroxide at 150180 F., and is then reacted with a stoichiometric amount of dimethyl sulfate at l-l80 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,656,380 Turner Oct. 20, 1953 

1. A METHOD OF REDUCING THE ACID NEUTRALIZATION NUMBER OF CRUDE PETROLEUM CONTAINING SUBSTANTIAL AMOUNTS OF NAPHTHENIC ACIDS WHICH COMPRISES TREATING THE PETROLEUN WITH AQUEOUS ALKALI TO CONVERT THE NAPHTHENATE ACIDS TO NAPHTHENATE SALTS, REACTING THE NAPHTHENATE SALTS IN SITUWITH AN ESTERIFYING AGENT SELECTED FROM THE GROUP CONSISTING OF LOWER ALKYL SULFATES AND LOWER ALKYL HALIDES TO PRODUCE A LOWER ALKYL NAPHTHENATE AND BY-PRODUCT SALTS SELECTED FROM THE GROUP CONSISTING OF ALKALI HALIDES AND ALKALI LOWER ALKYL SULFATES, AND TREATING THE PETROLEUM IN A DESALTER TO REMOVE BY-PRODUCT SALTS AND NATURALLY OCCURRING SALTS. 